Bypass valve

[Tonygeek]

Thanks again John. I have a BS4613 dial-type tyre pressure gauge, which is marked in 0.1 bar steps, so I used this (at the top of my rig to avoid getting water into it!). The water supply was from the mains via a good length of hosepipe and I had the valve discharging into a large bucket. A bit Heath Robinson and messy but I was able to control the pressure and flow by carefully opening the mains tap a little. I would say that the flows I was getting through the valve when it should have been shut were less than 5 l/min. I would need to spend time improving the set up in order to have any free hands to take measurements etc!

I am toying with the idea of trying to lap in the valve jumper to its seat with a little fine valve grinding compound as one used to with car engine valves. I am surprised they don’t use something a bit more resilient than the hard plastic for the valve jumper, which would have a better chance of giving a seal. Maybe they are not worried about something less than a complete seal?

 

[Johntheo5]

They shouldn't pass anything if shut, not even 0.2LPM, what's the point otherwise?, you may as well have that gate valve suggested above.

 

[Tonygeek]

I completely agree, John, but this is what I am faced with - both the new valve and the old one. Building regs require an automatic bypass and not a fixed one. Surely one that works is not asking too much of modern science?!

 

[Tonygeek]

I've been pondering a bit further and think I've come up with a simpler and probably better method for testing the removed bypass valve. But more of that later, as I've also been trying to get my head around what the boiler and pump will do in response to a high external system resistance, both during normal operation and during pump overrun.

Assume that just one heating circuit is in operation (DHW off) with the pump set to one of the constant pressure maps ……….. Presumably, as TRVs close and system resistance increases, the boiler output and pump speed reduce to maintain the set pressure and prevent its increasing. This can continue until the boiler is at its minimum output of 3kW and the pump speed is down to its minimum of 10%. But what happens if system resistance then increases further? As the pump can’t slow up any more, presumably the pressure must rise above the set ‘constant’ level until it reaches the ABV setting, thus opening the valve and limiting further pressure rise? There is also the question of maintaining minimum permitted flow through the boiler but do I take it that the boiler doesn’t actually monitor this itself?

In the case of pump overrun, when the system resistance suddenly becomes infinite on closure of the heating circuit motorised valve, the pressure must leap up and thus quickly open the ABV. I have noticed that during the overrun period the pump modulation is shown as being in the range 10%-20%, which ordinarily would generate a really low pressure but I guess it is all that is needed to generate the pressure required to keep the ABV open during this period?

I should be very grateful if someone more knowledgeable than me would be able to confirm whether I am on the right track here or, if not, would be able to put me right. Just as a reminder, my boiler is a Worcester Greenstar 8000 system boiler. All help much appreciated!

 

[Tonygeek]

Seems like everyone is too polite to comment on my resistance musings above!

Meanwhile, I have now retested my removed and serviced ABV (Honeywell DU144) using a simple gravity feed arrangement with a head of 1.43m, which equates to 140mb pressure. With the valve set to 400mb, the leakage rate was 100ml per minute, increasing somewhat at lower settings. However, it was quite sensitive to the setting being altered, with the valve sealing more or less well after disturbance.

My conclusion is that, because of the design with a hard plastic valve jumper only held against a brass seat by spring pressure, a perfect seal is unlikely. My observations in use, where the return water after passing around the 8.5m ABV loop is very little cooler than the flow temperature, result from the fact that the whole loop is insulated because it is also the primary loop for the DHW cylinder.

I therefore intend to remove the insulation from the return leg of the loop so that, when the pump goes into overrun, there is about 4m of 22mm pipe containing cooler water to help with removal of residual heat from the boiler. I am anticipating that this water will be significantly cooler than it currently is because of the low leakage flow through the ABV. I will still have the benefit of the flow leg of the loop being insulated for heating of DHW.

 

[Johntheo5]

Didn't "get" your Sat post until about 2 hours ago.

OK, first thing is, regarding re pump pressure rise against a closed valve, all centrifugal pumps will only rise to the max head at 100% with a closed valve, a 700mb pump will not rise above 700mb at 100%, since head is proportional to speed-squared, if the pump speed is not > 75.5% then the max closed valve pump head will be < 400mb (ABV setting).

You, or anyone, can play around with the "live" spreadsheet, also attached, below.

I have configured a few different spreadsheets based on a assumed boiler HEX head loss of 1M/16LPM and on MY 10 rad system of 3M/11LPM.....probably reasonably typical of a lot of systems.

Forgot to ask, is that ABV leakage rate from the one you removed previous to installing the present one??.

Hydraulic setting of 150mb = 52% speed, 189mb (closed valve pump head), and pump head of 170mb, Res head of 150mb, flowrate 7.8LPM.
Hydraulic setting of 200mb = 60% speed, 252mb (closed valve pump head), and pump head of 204mb, Res head of 200mb, flowrate 9.0LPM.
Hydraulic setting of 400mb = 85% speed, 506mb (closed valve pump head), and pump head of 488mb, Res head of 400mb, flowrate 12.7LPM.
Hydraulic setting of "100%"=100% speed, 700mb (closed valve pump head), and pump head of 625mb, Res head of 550mb, flowrate 15.0LPM.

 

[Johntheo5]

I've been pondering a bit further and think I've come up with a simpler and probably better method for testing the removed bypass valve. But more of that later, as I've also been trying to get my head around what the boiler and pump will do in response to a high external system resistance, both during normal operation and during pump overrun.

Assume that just one heating circuit is in operation (DHW off) with the pump set to one of the constant pressure maps ……….. Presumably, as TRVs close and system resistance increases, the boiler output and pump speed reduce to maintain the set pressure and prevent its increasing. This can continue until the boiler is at its minimum output of 3kW and the pump speed is down to its minimum of 10%. But what happens if system resistance then increases further? As the pump can’t slow up any more, presumably the pressure must rise above the set ‘constant’ level until it reaches the ABV setting, thus opening the valve and limiting further pressure rise? There is also the question of maintaining minimum permitted flow through the boiler but do I take it that the boiler doesn’t actually monitor this itself?

In the case of pump overrun, when the system resistance suddenly becomes infinite on closure of the heating circuit motorised valve, the pressure must leap up and thus quickly open the ABV. I have noticed that during the overrun period the pump modulation is shown as being in the range 10%-20%, which ordinarily would generate a really low pressure but I guess it is all that is needed to generate the pressure required to keep the ABV open during this period?

I should be very grateful if someone more knowledgeable than me would be able to confirm whether I am on the right track here or, if not, would be able to put me right. Just as a reminder, my boiler is a Worcester Greenstar 8000 system boiler. All help much appreciated!

Are you sure the pump speed is down to 10% at 3.0kW minimum boiler output, if so, then its maximum head is 7.0mb on pump overrun with a pumphead/reshead/flowrate of 6.2mb/5.3mb/1.5LPM, this will result in a boiler dT, of, 3.0*860/60/1.5, 28.7C.

 

[Tonygeek]

Thank you again John. Responding to your points made and asked in your two posts above in order:

Pump pressure against closed valves: Given the pump modulation of 20% I am observing during pump overrun, what is the point of the ABV and the overrun if the pressure generated is nowhere near the 400mb required to open it?

The ABV leakage rate of 100 ml/min at 400mb setting that I measured yesterday was for the old removed valve that I cleaned and serviced. The observations I am reporting for boiler and pump performance are for my system with the new valve.

Where I mentioned minimum boiler output of 3kW and minimum pump speed/modulation of 10% in my musings post #124 above, I was just noting that these are the lowest values at which the boiler and pump can operate. However, I have just watched the boiler info display at the moment of heating circuit shut-off and the “pump mod.” value went from 30% during operation to 20% during overrun and stayed at 20% throughout the overrun period.

I have assumed that “pump mod,” as displayed equates to % of maximum pump speed/performance. Given that the pump is set on a constant pressure map but that the pump mod. figure varies during operation, I have taken this to indicate that it is the pump speed that is varying and being displayed accordingly.

I hope this answers your queries but I am left wondering how the overrun function can fulfil its purpose of relieving pump pressure and dissipating residual heat from the HEx if there is not enough pressure to open the ABV?

My brain is starting to hurt!

 

[Johntheo5]

Thank you again John. Responding to your points made and asked in your two posts above in order:

Pump pressure against closed valves: Given the pump modulation of 20% I am observing during pump overrun, what is the point of the ABV and the overrun if the pressure generated is nowhere near the 400mb required to open it?

The ABV leakage rate of 100 ml/min at 400mb setting that I measured yesterday was for the old removed valve that I cleaned and serviced. The observations I am reporting for boiler and pump performance are for my system with the new valve.

Where I mentioned minimum boiler output of 3kW and minimum pump speed/modulation of 10% in my musings post #124 above, I was just noting that these are the lowest values at which the boiler and pump can operate. However, I have just watched the boiler info display at the moment of heating circuit shut-off and the “pump mod.” value went from 30% during operation to 20% during overrun and stayed at 20% throughout the overrun period.

I have assumed that “pump mod,” as displayed equates to % of maximum pump speed/performance. Given that the pump is set on a constant pressure map but that the pump mod. figure varies during operation, I have taken this to indicate that it is the pump speed that is varying and being displayed accordingly.

I hope this answers your queries but I am left wondering how the overrun function can fulfil its purpose of relieving pump pressure and dissipating residual heat from the HEx if there is not enough pressure to open the ABV?

My brain is starting to hurt!

IF the pump is mapped to one of the constant residual pressures then if set to 150mb the minimum pump speed should not fall below 46.3%, if set to 200mb, it should not fall below 53.4%, if set to 400mb. it should not fall below 75.6%. It appears to be mapping to the thermal output of the boiler. Next time the boiler is on note the pump modulation and the pump speed, also post the hydraulic setting you have set.
A ABV will really only work properly with a pump on a fixed speed setting, would suggest setting the ABV to 0.5index (500mb) and change the hydraulic setting to enable the setting of a fixed pump speed to 85/86% which will give a closed valve head of 506/518mb, sufficient to operate the ABV to give enough flow on pump overrun, when heat again required, then the ABV will/should shut once the flowrate is > 4.0LPM.

 

[Tonygeek]

Thank you John. The pump map is definitely set to ‘Delta-P dependant 1’, ie 150mb. The other info displayed re the pump is:
Control mode: ‘Heat demand’ (the alternative is ‘Energy saving’); Block time: ‘0 sec’; Overrun: ‘2 min’.
In ‘Hydraulics’ the info is: ‘Low loss header: Off’; ‘DHW configuration: 3-way valve installed’ (but I do not believe we have a 3-way valve - all the zone valves, including for DHW, are external to the boiler); ‘HC1 configuration: no dedicated pump installed’; ‘Pump configuration: Heating pump’.

The pump Min O/P and Max O/P values currently set at 20% and 100% are only displayed when in the ‘Output dependant’ map option but, once set there, appear to apply to the Delta-P dependant maps also.

I watched the pump modulation info at heating start-up this morning and, as the boiler wound itself up over the course of some minutes, the pump modulation climbed to 49% as the boiler output climbed to 17kW and then stayed at 49% as the boiler subsequently throttled back to 8kW.

I don’t need the ABV to open during normal operation because both heating circuits have a couple of rads without TRVs in the areas where the room stats are (plus a couple of towel rails also without TRVs). It’s just for the pump overrun that it is needed but the pump control seems to drop the modulation to 20% (currently the set minimum) during the overrun period.

 

[Johntheo5]

Thanks Tony,
It must be a characteristic of WB boilers to drop the pump speed to 20% on overrun. If happy with that 150mb setting then just set the minimum pump speed to say 43% and see if the pump speed only falls to this (43%) on pump overrun, if so, set the ABV to slightly < 0.15,say "0.13".
still not ideal but can be fine tuned to work pretty well I would think. What I've done on a few installations where, like yours, only require a bypass on pump overrun, is install a NO (normally open) motorized valve instead of a ABV, which only opens when all zone valves are (de energised) closed, this then only recloses when any zone valve is energised open.

 

[Tonygeek]

I've been having exactly the same thought, John, that an MV interlocked as you suggest would be a more positive and less tricky solution than an ABV that neither shuts off properly or will definitely open when required! Is the interlock signal/power needed generally easy to obtain from the other valves?

 

[Johntheo5]

Can't remember how I did it now but your boiler will have a permanent live and a switched live, the switched live will be from terminal 10, the boiler then tells the pump to run/overrun (not like the schematic version, below), when all the zone valves close then there is no voltage at terminal 10 so probably connect the by pass MV between this terminal and neutral, it should then close once any zone valve opens but will not open until all zone valves are closed.

 

[Tonygeek]

Thank you John. That is helpful. I may have a couple of Honeywell ABVs for sale 'ere long!

 

[denso13]

I'm not the first to say it, fit a gate valve.